414 Einthoven and Jolly 



Grijns and Noyon.s' ^ minimum is 



0-95 to 26 X 10-18 g. cal. = G-4 to 11 x IQ-^o erg. 



Von Kries^ gives 



3-1 to 6-2 X 10-18 g. cal. = 1-3 to 26 x lO-^o erg. 



In fig. 23 we see the development of a fairly strong photo-electric 

 reaction to a stimulus which uses 30'5 x 10-^^ g. cal. per sec., which lasts for 

 484 sec, and thus possesses a total energy of 1*47 x lO-^^ g. cal. In all 

 probability there may be obtained a perceptible galvanometric deflection 

 with a stimulus 10 times, and perhaps even 100 times weaker, but our 

 experiments have not extended further in that direction. Nevertheless, it 

 is sufficiently clear that for the photo-electric reaction of an isolated frog's 

 eye, there is required much more light than for the development of a 

 light perception in the human eye. 



On the other hand, a comparison of the isolated frog's eye with the most 

 sensitive bolometer constructed by human skill is greatly tx) the dis- 

 advantage of the latter. 



V. The Relation between the Energy of the Stimulation 

 AND the Energy of the Reaction. 



Generally speaking, the rule holds good that for moderate and strong 

 stimuli the energy of the reaction increases or decreases much less rapidly 

 than t}\e energy of the stimulus. Perhaps it is to be expected that here the 

 Weber-Fechner law is true ; that is to say, that when the stimuli increase 

 in geometrical progression the reactions increase in arithmetical progression. 

 This should be investigated for each of the three substances separately, 

 which has not so far been done. 



If very weak radiations are employed, the above-mentioned law does not 

 hold good, as is sufficiently evident from the series of four photographs, figs. 

 11, 12, 13, and 14. Here the energy of reaction is increasing considerably, 

 whereas the increase in the energy of radiation is but small. If the 

 radiations are weakened still more, the reactions decrease relatively quickly 

 in energy, and it is reasonably to be expected that under these conditions 

 all perceptible galvanometric deflections will soon fail. The results of the 

 experiments are in complete agreement with this expectation. 



In reference to tliese problems, we may recall the work of de ilaas,^ who 

 has confirmed the Weber-Fechner law within wide limits; but de Haas 

 used a slowly deflecting galvanometer, so that the separation by him of the 

 action of three substances w^as impossible. Presumably, his measurements 

 with stronger stimuli relate principally to the action of the third substance, 

 while those with weaker stimuli may have had reference to the combined 

 action of the second and third substances. 



^ Engelmann's Arch. l. Physiol., p. 25, 1905. ^ Lqc (.jt. 3 Lq^. cit. 



